Not that I've played GT5 since GT6 came out (too busy trying not to spin out once a lap), but GT5's physics in my opinion were the best of any game I've ever played. It's what kept me coming back for nearly 3 years. It just flowed so nicely. It was predictable. It felt realistic and alive, but never was it too hardcore. GTA V for example, you can hit a small pebble twice, same speed, same angle. First time, the car will hop over it, second time the car will flip and the driver's side wheel will become jammed. GT6, MR cars are more realistic. More fun? Not for me.
With the right set-up, any GT5 car could be driveable. You could tame an LP 640 and feel like a total hero. Yes, I am a Racing soft tyre user. I retain that it's fun on GT5. On GT6, it's more challenging depending on the car. The GT40 Mk I - on RS tyres, it's like watching your Dad dance. It's so awkward. So for that reason, GT5 still holds a special place in my game collection, along with Borderlands 1. Those are the two games I've spent the most time playing out of all of my games.
R.I.P, GT5 online, where I logged a good lot of hours of my time.